1. Field of the Invention
The present invention relates to a granulation process and provides for new and improved apparatus eminently useful for production of high-strength urea granules from low-strength urea prills. It will be appreciated by the chemical fertilizer industry and especially those skilled in the art of urea granulation that the practice of the present invention will result in the production of a predetermined and closely sized, urea-granular product of superior strength that is highly desirable and necessary for the production of nonsegregating bulk blends, sulfur-coated urea, and for direct application to the field. It will also become readily apparent to those skilled in this art that the practice of the present invention results in significant cost benefits in the commercial production of granular urea while utilizing low-cost, surplus urea prills to produce a highly marketable product. Still another benefit derived from the practice of the present invention allows the chemical fertilizer industry to increase the rate of production of granular urea to accommodate the seasonal demand for agricultural nitrogen inherent to the commercial fertilizer market.
A principal consideration relating to the practice of the present invention is the selection and use of methods and/or means wherein the modus operandi comprises the use of low-strength urea prills as substrate to produce high-strength granular urea.
2. Description of the Prior Art
There is a special need within the chemical fertilizer industry for efficient/economical, continuous processes and apparatuses capable of high-capacity production of high-quality urea fertilizer in predetermined, closely sized granular form. It will become apparent and appreciated by those versed in this art that the present invention is particularly well suited for meeting that special need and furthermore that the present invention is also readily adaptable on small scale to the requirements of specific industries such as, for example, the production of coated pharmaceuticals, candies and/or other similar applications. Therefore, the discussions and disclosures in the following sections describe the prior art and the application of the present invention to the production of fertilizer, both for the standpoint of granulation, i.e., gradual increase of the undersize nuclei to the desired product size by successive layering (accretion) and coating or incorporation of dissimilar fertilizer compounds such as, for example, incorporating primary and secondary plant nutrients within the granules or otherwise applying to the surface of substrate particulates permeable or semipermeable coatings of natural or synthetic polymeric compounds, oils, waxes, asphaltic and/or paraffin mixtures, and combination of such material for the purpose of the production of controlled release and other fertilizer products of proven desirable characteristics which can be produced, by my noncomplicated methods and equipment, at reasonable costs.
U.S. Pat. Nos. 3,117,020, Fabris et al. Jan. 7, 1964; 3,165,395, McCamy et al. Jan. 12, 1965; and 3,211,522, Shurter et al, Oct. 12, 1965, disclose various methods and means for the granulation of undersized fertilizer compound nuclei (recycled fines from screened product) by spraying a hot, concentrated aqueous solution or a nearly anhydrous melt of the compound into a rolling bed of the nuceli in an inclined, rotating pan. Although there is some coating action, most of the granulation, or increase to product size, is accomplished by agglomeration, or sticking together, of a number of the nuclei by the solidifying melt or solution. The granules are relatively rough and irregular, compared with layer granulation (see later section), and coating per se is minimal. The unit has little application as a true coating unit because of its natural classification action.
In U.S. Pat. No. 2,815,376, Knowlton et al. Dec. 3, 1957, there is disclosed a process for the granulation of fertilizer compounds by the spraying, or simple mixing, of the undersized fertilizer compound nuclei with a hot solution or melt of the compound in a paddle mixer, i.e., a blunger or pug mill. Again, particulates are formed by agglomeration of several nuclei into a single, larger particle bound together by the solidifying melt. Little coating action is achieved and the particles are rough and irregular.
It has also been shown that granulation, predominantly by a true coating or layering action, can be accomplished by spraying the hot solution or melt of a fertilizer compound into a fluidized bed comprised of undersized compound nuclei. For instance, as disclosed in U.S. Pat. No. 2,600,253, Lutz, June 10, 1952, ammonium nitrate or ammonium sulfate fertilizers are produced by reacting ammonia and nitric acid or sulfuric acid in a fluidized bed of undersized ammonium nitrate or ammonium sulfate particles. In other applications, principally in Europe, a hot melt or concentrated solution in the compound is sprayed into the fluidizing gas (hot air) at the bottom of the fluidized bed. The fluidized bed does achieve truly random motion of the substrate particles, and therefore results in a homogeneous mass with respect to particle size which is a consideration so necessary in uniformly coating particles of varying sizes. However, the fluidization process is inherently costly, it requires close control, and it does not permit visual examination of the sprays or product in the coating section.
In the fertilizer industry, granulation, i.e., increase in particle size from undersize (recycled fines) to product size by coating, and coating for the purpose of imparting special characteristics to the fertilizer substrate, such as controlled release or anticaking properties, is most widely practiced in essentially horizontally disposed rotary drums either with or without internal lifting vanes or flights. In U.S. Pat. No. 3,398,191, Thompson, et al, Aug. 20, 1968, disclose the granulation of urea or ammonium nitrate by spraying an essentially anhydrous melt (98-99.5 percent) of the coating compound from multiple spray heads spaced at intervals along the entire length of the coating section onto a rolling bed and into a showering curtain of undersized nuclei (recycled fines) maintained in motion by the rotation of a slightly inclined (from horizontal) rotating drum equipped with longitudinal lifting vanes or flights specially designed to form continuous longitudinal curtains of falling particles that move in succession across the entire cross-sectional area of the contact or coating zone of the drum in a manner familiar to those versed in the art of horizontal rotary drum coolers and dryers. Transverse retaining rings or dams at the feed end and discharge end of the coating section maintain an adequate depth of bed. Cooling air (ambient temperature) is drawn countercurrently through the showering curtains of falling particles to cool and solidify the layers of melt on the nuclei. The drum is extended beyond the contact zone to a cooling zone equipped with the lifting flights but without spray heads with which to further cool the product with such countercurrent flow of air.
Other essentially identical examples of this form of prior art featuring the falling curtain across the full cross-sectional area of the rotary drum are disclosed in U.S. Pat. Nos. 3,092,489, Smith, June 4, 1963; 3,277,789, Tytus et al, Jan. 4, 1966; and 3,232,703, Thompson et al, Feb. 1, 1966.
The falling curtains of particles across the full cross-sectional area of the drum, as described in the above prior art, approaches the degree of random motion of substrate, and therefore of homogeneity with respect to particle size, that is so important to the uniform coating of a mass of particles of different sizes, but it is now believed that the arrangement of spray heads within the shower of falling particles, and therefore in actual contact with many of the falling particles as a result of the falling curtains across the entire cross-sectional area of the drum, has certain serious disadvantages. Among the most serious of these disadvantages are (1) the lack of control of the spray distance, i.e., the distance that the individual droples of atomized liquid spray travels before impinging upon the moving substrate particles; some of the particles fall on each spray head, some immediately in front of it, some fall at the optimum distance, and some fall well beyond the optimum distance but are still contacted by the spray. This leads to agglomeration of the substrate particles when too short, or too rough--ineffective coatings, when too far; (2) the actual contact of many of the falling particles with the hot spray heads leads to melted substrate, which drips onto the substrate bed, causing serious agglomeration of some of the substrate; (3) visual monitoring of the invidual spray operation of impossible and (4) dusting is serious when the entire section of the rotating drum is filled with falling particles, dust formed by attrition, and solidified spray mist, all of which can be carried from the system by the cooling or heating air flowing through the coating unit. This dust problem substantially increases antipollution equipment requirements.
The coating procedure, as taught in U.S. Pat. No. 3,295,950 Blouin et al. Jan. 3, 1967; U.S. Pat. No. 3,342,577, Blouin et al, Sept. 19, 1967; and U.S. Pat. No. 3,903,333, Shirley et al, Sept. 2, 1975, are almost identical in nature, i.e., the directing of the atomized coating material only onto the rolling bed of substrate in a horizontal rotary drum having a relatively smooth interior with no lifting vanes or flights as is shown and provided for in U.S. Pat. No. 2,741,545, Nielsson, Apr. 10, 1956. The latter art, i.e., Shirley U.S. Pat. No. 3,903,333, discloses certain improvements in the former which, according to the example data disclosed, does result in somewhat more uniform coatings than those of the former art, i.e., Blouin U.S. Pat. No. 3,295,950.
However, both practice essentially the same approach as described above and, therefore, both suffer from the same serious disadvantage that precludes a truly homogeneous moving bed of particles of different sizes and therefrom true uniformity of the coating, namely, the segregation by particle size of particles of varying sizes that occurs in a smooth, horizontal rotary drum. This segregation or demixing is well documented in the extensive work of Campbell et al [Chemical Engineering 73(19), 179-185 (Sept. 12, 1966)]and McDonald et al [British Chemical Engineering 7(10), 749-753 (October 1962--Part I, ibid 7(11), 823-27 (November 1962)--(Part II; and ibid 7(12), 922-23 (December 1962)--Part III]. Although the degree of demixing or segregation that occurs in a smooth drum may be reduced by proper choice of operating and equipment variables such as bed depth or degree of drum loading, drum speed (expressed as percent of critical speed, the critical speed, i.e., revolutions/minute, of a smooth drum being defined as 76.5 /.sqroot.D, where D=drum diameter in feet), and ratio of drum diameter to drum length, it cannot be eliminated. As a result, smaller particles tend to segregrate from the larger particles by going to the point of lowest particle velocity, namely, the center of the cross-sectional area of the bed and pass on through the drum without coming to the surface of the bed. This, of course, prevents these particles from being coated by the liquid spray.
The problems associated with segregation of particles by size within a horizontal rotating drum during application of coatings and/or granulation have been significantly reduced in the art disclosed in U.S. Pat. No. 3,877,415, Blouin, Apr. 15, 1975. The apparatus in this disclosure provides for a near homogeneous (with respect to particle size) dense mass of sized particles in random motion so that highly uniform coatings of the same or of different solids can be applied to each particle by conventional spray-coating with the liquefied coating material(s). The apparatus is a horizontal rotary drum containing lifting flights. A novel deflector pan is fixed in space inside the upper section of the drum which deflects particles falling from the lifting flights to the side of the drum where they form a narrow, dense falling cascade. The coating material is sprayed onto the cascading particles, preferably as they free-fall after leaving the lower edge of the pan. However, if desired, some or most of the coating material may be directed onto the top edge of the moving bed including the juncture of the cascade therewith. Also see U.S. Pat. No. 3,991,225, Blouin, Nov. 9, 1976.
Further improvements in the prior art of Blouin's disclosure supra were revealed in the procedures taught in U.S. Pat. No. 4,213,924, Shirley et al, July 22, 1980; U.S. Pat. No. 4,424,176, Shirley et al. Jan. 3, 1984; and U.S. Pat. No. 4,506,453, Shirely et al, Mar. 26, 1985. In these further improvements of Shirley et al, there is disclosed an improved process for the granulation or coating of hygroscopic or nonhygroscopic materials where melt is sprayed onto cascading granules of common or uncommon substrate in an enclosed vessel and where the heat given off by solidification of the melt is absorbed by evaporation of water. The water is atomized into the granulator as an extremely fine mist and evaporation is effected without impingement of such mist on either the granules or granulator internals. It was further disclosed by Shirley et al supra that the installation of two or more inclined deflector pans in step fashion in the rotary granulation drum allows the substrate elevated by lifting flights from the cascading bed in the drum to fall onto the pans. Material cascading from the pans form the upper and lower falling curtains of substate. Molten or highly concentrated urea solution is sprayed with a high degree of precision horizontally onto the lower falling curtain of substrate usually throughout the entire length of the lower falling curtain. Air cooled by the evaporation of water, which now and later will be referred to as evaporative cooling, is forced upon and through the uppermost falling curtain of substrate (recycle and nuclei) by several internally-mounted propeller fans to effect the high degree of cooling and removal of heat released in the rotary granulation drum by the solidification of molten or highly concentrated urea solution. For purposes of teaching, disclosing, and claiming the instant invention the full teachings and disclosures of Blouin U.S. Pat. No. 3,991,225 and 3,877,415 as well as Shirley U.S. Pat. Nos. 4,506,453 and 4,424,176 supra are herewith and hereby incorporated herein by reference thereto.
Those skilled in the art are well aware of heat transfer technology as it applies to fluid beds and spouted fluid beds such as have been disclosed in U.S. Pat. No. 4,219,589, Niks et al, Aug. 26, 1980, and in U.S. Pat. No. 4,217,127, Kono et al, Aug. 12, 1980, respectively. Fluid-bed technology is recognized to be one of the best heat transfer means between a gas and solid particles. The heat transfer rates within the bed are exceptionally high.
As disclosed in the teaching of Shirley et al supra, many of the principles of fluid bed were emulated herein in part to achieve extremely effective means of heat transfer. It is also an objective of the present invention to make use of fluid-bed principles to achieve efficient heat transfer and at the same time to eliminate the less desirable features in the art as practiced by Shirley et al. In this respect, the present invention borrows from the fluid bed the principle that gas blowing through suspended solid particles in a more or less dense phase, as in a fluid bed, is the best means of contact for heat transfer purposes and not as gas contact occurs in long rotary drums where gas flow is axial sometimes passing through, but mostly flowing parallel, to the showers of falling solid particles. More specifically, the present invention approaches the principles of fluid bed in the rotary granulation drum by design and action of the lifting vanes or flights and by means of a specially designed multipurpose assembly which comprises upper and lower deflector pans and a cooling-air distribution manifold to form the cascading recycle and nuclei into an upper and lower fixed falling curtain wherein essentially anhydrous urea solution or melt is sprayed hydraulically with great precision at low to moderate pressure at right angles to and along the full length of the lower curtain. Cooling air drawn from outside the facility by a single blower is forced upon and through the upper falling curtain through use of an air-distributing manifold located between and attached to the upper and lower inclined deflector pans and extends the full length of the falling curtain of substrate.
The embodiment of evaporative cooling in the procedures disclosed by Shirley et al supra is incorporated in 14-ton-per-hour falling-curtain urea-granulation plant located at the Tennessee Valley Authority's National Fertilizer Development Center, Muscle Shoals, Ala. Although the plant has been in operation since July 1983, the technology has not been well received by those versed in the art for, among other reasons, the equipment requires unusually high maintenance, the process is complicated and difficult to operate, and when it is operated improperly it causes caking and buildup on the shell and other internal parts of the rotary granulation drum.
Since of the objectives of the present invention is to develop a both a process and granulation apparatus which demonstrates reliability and simplicity in operation, the above-mentioned principle of evaporative cooling is not utilized therein because of its inherent disadvantages. Additionally, the degree of heat transfer and removal cited by Shirley et al for evaporative cooling is not needed in the practice of the present invention which now provides for heat transfer and removal from the rotary granulation drum by a much simpler and much more reliable method and/or means.